Pure Fluorine Intercalation into Brownmillerite Oxide Thin Films by Using Ionic Liquid Gating

Electrolyte gating, exemplified by ionic liquid gating (ILG), has emerged as a promising technique for modifying the physical properties of transition-metal oxides through cation intercalations, such as H+ and Li+. However, the anion insertion by the ILG remains largely unexplored, except for O2–. In this study, we have developed an ILG-based F– insertion method to allow oxidative fluorine insertion without a loss of oxide ions. Employing a solution of tetramethylammonium fluoride tetrahydrate in 1-methyl-1-propylpyrrolidinium bis-trifluorosulfonyl-imide as an electrolyte, fluoride ions were electrochemically inserted into brownmillerite SrCoO2.5 films, resulting in the formation of a SrCoO2.5F0.5 perovskite with the Co3.5+ state. Similar to its oxide counterpart La0.5Sr0.5Co3.5+O3, the obtained SrCoO2.5F0.5 is a ferromagnetic metal with a Currie temperature (TC) of ∼110 K. Notably, this behavior was not observed in any SrCoO3–xFy compounds prepared using conventional reactions. Additionally, this technique was applied to SrFeO2.5 brownmillerite, leading to SeFeO2.5F0.5 perovskite, which suppresses a charge disproportionation as found in the oxide counterpart La1–xSrxFeO3. The ILG-based fluorination reaction developed in this study opens new avenues for the exploration of functional oxyfluorides from oxygen-deficient oxide hosts.